1. Field of the Invention
The present invention relates to one-dimensional or two-dimensional photoelectric conversion apparatus and image reading apparatus used in facsimile machines, digital copiers, X-ray image pickup apparatus, or the like, and more particularly, the invention concerns an element separation structure used therein.
2. Related Background Art
A reading system using a reduction optical system and a CCD type sensor has been used heretofore as a reading system in facsimile machines, digital copiers, X-ray image pickup apparatus, or the like, but with the development of photoelectric conversion semiconductor materials typified by hydrogenated amorphous silicon (hereinafter referred to as a-Si), recent development is remarkable in so-called contact type sensors in which photoelectric conversion elements and signal processing sections are formed in a large-area substrate and which is arranged to read information on an information source using an optical system of magnification of unity.
FIGS. 7A to 7C are cross-sectional views to show structures of conventional photoelectric conversion apparatus as disclosed in Japanese Laid-open Patent Application No. 54-146681.
FIG. 7A shows a cross-section of an array composed of two elements, in which reference numeral 1 designates an n.sup.+ type Si substrate, 2 a high-resistance n.sup.- layer, 3 a plurality of isolated zones of p.sup.+ layer, and 4 electrodes. As shown in the drawing, the apparatus is normally reverse-biased, and load resistors R.sub.1, R.sub.2 are connected between the p-type electrodes 4 and a bias power supply. Among carrier pairs occurring near the border between the two elements, holes are attracted to the p.sup.+ zone 3 by an electric field, but some of them also flow into the adjacent element because of diffusion due to concentration gradient, thereby causing crosstalk. This did not allow the distance between elements to be decreased to below several tens of .mu.m.
FIG. 7B is a cross-sectional view to show another conventional example, in which reference numeral 21 designates a high-concentration n.sup.+ type silicon substrate, 22 a high-resistance and low-concentration n.sup.- silicon layer having the same conduction type as the substrate does, and 23, 24 a plurality of p.sup.+ type regions of the opposite conduction type to that of the n.sup.- layer 22, which are separated from each other, thus forming a plurality of elements (photosensor elements) 25, 26 in p-n.sup.- -n.sup.+ structure. Further, numeral 27 denotes a high-concentration n.sup.+ region formed relatively deeply, and 28, 29 low-concentration p.sup.- regions formed relatively shallowly.
In the elements of this structure, curved lines indicated by dotted lines in the n.sup.- layer 22 indicate the electric field. Horizontal components of the electric field prevent the carrier pairs occurring near the border between the p.sup.+ regions 23, 24 with reception of incident light from diffusing to the adjacent element as the holes are subject to drift to the p.sup.+ regions 23 or 24 of the element 25 or 26 which has received the incident light. The holes near the n.sup.+ layer 27 at the border and near the border with the substrate 21 also tend to be separated by the horizontal components of the electric field.
FIG. 7C shows an example in which the n.sup.+ region 27, shown in FIG. 7B, is formed still more deeply down the surface of the substrate 21. When a part of the field is connected with the substrate 21 through the n.sup.+ layer 27 in this way, element separation becomes better.
FIG. 8 is a schematic cross-sectional view to show the structure of a semiconductor photodetector as another conventional example disclosed in Japanese Patent Publication No. 64-6547.
In the drawing, an n.sup.- layer 72 is epitaxially grown, for example, on a p.sup.+ type semiconductor layer 71 which serves as a substrate, and between p.sup.+ layer 73 and n.sup.+ layer 74 there is provided a p.sup.- layer 70 having an impurity concentration higher than that of the n.sup.- layer 72 and not higher than one fifth of that of the p.sup.+ layer 73.
Numerals 75, 76, 77 denote electrode leads, and reverse bias V.sub.e is placed through the electrode leads 75, 77 between the n.sup.+ layer 74 and the p.sup.+ layer 71 which is the substrate, thereby forming respective diodes.
Under such bias setting, the carriers (electrons) diffusing horizontally go into the n.sup.+ layer 74 to recombine therewith, thereby obtaining photodiodes in an array structure nearly perfect in signal separation between the elements and very low in crosstalk.
However, in the cases where, in the substrate with plural photosensor elements formed therein, element separation is effected by the semiconductor regions of the same conduction type as that of substrate in order to collect the horizontally diffusing holes as described in the above conventional examples, there is a problem to be solved in that crosstalk characteristics between elements are still inadequate.
There is another problem, that injected carriers from a peripheral circuit system to the substrate become false signals, which facilitates degradation of characteristics in FPN (Fixed Pattern Noise).